1. Field of the Invention
The present invention relates generally to surgical instruments, and in particular to microtubes, and methods of processing them.
2. Background Discussion
Microtubes are fine tubes having very small outer and inner diameters. As used herein, the term “microtube” refers to a fine tube having an outer diameter ranging from about 4 to about 12 mil (1 mil= 1/1000 in or 0.0254 mm).
Microtubes can be used in a number of applications in the medical industry, such as infusion of fluids, drainage of fluids, delivery of anesthesia, and the like. In order to reduce patient discomfort and other complications associated with the removal of the microtubes, it is desirable for such, microtubes to be made from absorbable materials so that the microtubes may be left in the body and eventually absorbed within the body. Further, in order to minimize the amount of material that must be absorbed and metabolized by the body, it is desirable that wall thickness of the microtubes be as small as possible. Finally, when such microtubes are utilized for the infusion of fluids or the delivery of anesthesia, controlling the flow rate and delivery of fluids through the microtube for extended periods of time is essential. As a result, microtubes used for these purposes must have uniform inner and outer diameters and cross section throughout the entire length thereof.
However, production of uniform absorbable microtubes has always presented difficulties that are not easily overcome. For example, it is very difficult to produce absorbable microtubes having a variation of outer diameter of 0.15 mil or less, which is required for the control of a uniform flow of a fluid within 15% of a target flow rate. Maintaining dimensional stability of the microtube upon heating, sterilizing, loading, handling or implanting has also been a challenge because of the small diameters and thin walls of the microtube. Additionally, consistent production of a high strength and flexible microtube having such uniformity and dimensional stability is also problematic.
U.S. Pat. No. 4,720,384 teaches a process for preparing a hollow tube drug delivery system, where a polymer solution or suspension of polyolefins, polyurethanes, ethylene-vinyl acetate copolymers, polyvinyl alcohols, or blends of water-soluble polymers with some of the aforementioned polymers, was extruded through an annular orifice to form hollow core. A drug solution was simultaneously extruded into the hollow core to form a drug encapsulated tubular system. The extruded tubular system was coagulated under conditions to minimize orientation and to create pores in the polymeric tube wall. However, since the hollow tube has no molecular orientation and has pores in the wall, such a tubular system would be very weak or brittle and easily kinked or broken upon bending or loading of a force. Further, this reference is silent with respect to a microtube made from absorbable materials and the uniformity of the inner and outer diameters and cross section of the hollow tube.
JP07156251 describes production equipment for making small diameter tubes where the range of fluctuation in the outer diameter, inner diameter, wall thickness of the tubes are held to a minimum. This reference describes the use of a protective member at the output portion of the extruder head in order to prevent the extruded tube from being affected by wind. However, this reference is silent with respect to making of an oriented absorbable microtube with the strength and the dimensional stability thereof.
U.S. Pat. No. 5,100,379 describes a microcatheter having improved tensile strength so as to materially inhibit if not completely preclude the danger of breakage on removal from the body. The microcatheter is prepared by stretching or elongating a tubular article of greater diameter to molecularly orient the tubular walls while reducing the outer diameter to the desired microcatheter size of about 9 mil (or 0.229 mm). Nylons, polyurethanes and polyolefins are described as being suitable polymeric material to be employed to prepare the tubing. However, this reference is silent with respect to a microcatheter made from absorbable materials, and the dimensional stability of the microcatheter upon loading, heating, sterilizing, handling or implanting. While elongating and thereby molecularly orienting a polymeric catheter usually leads to a higher breaking strength, a mechanically oriented and stretched microcatheter has a very high potential to shrink, i.e., has low dimensional stability, upon exposure to heat or a high temperature during sterilization or storage, or after implantation in a human body.
U.S. Pat. No. 3,630,824 teaches a process to make extruded and stretched industrial monofilaments having a outer diameter of about 31.5 to 315 mil, and an inner diameter that is about 0.1 to 15 percent of the outer diameter. These industrial monofilaments are formed from fiber-forming thermoplastic polymers, for example, by spinning or extruding the polymer melt from a spinning head that is mounted in a vertical extrusion position. Such monofilaments are designed to be of a high-loading capacity, which is achievable by reducing the hollow cross section of the monofilament to less than 15 percent of the total cross section of the monofilament, thereby permitting one to achieve nearly the same loading capacity as a completely solid monofilament. This references discloses that the fine longitudinal channel located on the central axis of the monofilament apparently causes uniform shrinkage over the entire length of the monofilament upon cooling of the monofilament after spinning or extrusion. Further, this reference states that a second stage of stretching in a hot air zone develops the tensile strength in the large annular solid portion of the monofilament, while retaining a substantially uniform fine concentric channel which is so reduced in size that the total tensile strength is only slightly reduced from that of a corresponding filament having a completely solid cross section. However, this reference is silent with respect to a microtube made from absorbable materials and the dimensional stability of the hollow monofilament.
Therefore, there remains a need for producing a microtube from absorbable materials with uniform inner and outer diameters and cross section throughout the entire length thereof, superior dimensional stability and high strength, which is capable of controlling flow rate of therapeutic fluid without a rate-controlling device.